Abstract

A new method has been developed to predict acoustic scattering by weakly scattering objects with three-dimensional variability in sound speed and density. This variability can take the form of inhomogeneities within the body of the scatterer and/or geometries where the acoustic wave passes through part of the scattering body, into the surrounding medium, and back into the body. This method applies the distorted wave Born approximation (DWBA) using a numerical approach that rigorously accounts for the phase changes within a scattering volume. Ranges of validity with respect to material properties and numerical considerations are first explored through comparisons with modal-series-based predictions of scattering by fluid-filled spherical and cylindrical fluid shells. The method is then applied to squid and incorporates high resolution spiral computerized tomography (SCT) scans of the complex morphology of the organism. Target strength predictions based on the SCT scans are compared with published backscattering data from live, freely swimming and tethered squid. The new method shows significant improvement for both single-orientation and orientation-averaged scattering predictions over the DWBA-homogeneous-prolate-spheroid model.

The authors would like to thank Dr. Darlene Ketten at Woods Hole Oceanographic Institution and the colleagues in her group, particularly Julie Arruda, R.T.(R) who conducted the CT scans of squid presented here and Sr. Research Asst. Scott Cramer who aided in the handling, transport, and preservation of the specimens. The squid specimens used in this study were identified and provided by Ed Enos, Superintendent of the Aquatic Resources Division at Marine Biological Laboratory. We would also like to mention Dr. Roger Hanlon who provided great perspective concerning squid. Finally, the guidance and literature provided by Dr. Kohji Iida were most helpful in the comparisons with measured scattering by squid conducted in this research. Funding of this research was provided, in part, by the Office of the Oceanographer of the Navy and the Academic Programs Office at the Woods Hole Oceanographic Institution.